092  
FXUS02 KWBC 191553  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1153 AM EDT SUN AUG 19 2018  
 
VALID 12Z WED AUG 22 2018 - 12Z SUN AUG 26 2018  
   
..PATTERN OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES
 
 
THE FORECAST REMAINS ON TRACK FOR THE WELL-ADVERTISED PATTERN  
CHANGE EXPECTED TO OCCUR NEXT WEEK INTO THE WEEKEND. A STRONG  
RIDGE ALOFT BUILDING BETWEEN ALASKA AND HAWAII WILL LIKELY  
ENCOURAGE A DOWNSTREAM MEAN TROUGH TO BECOME ALIGNED FROM WESTERN  
CANADA TO NEAR THE NORTHERN CALIFORNIA COAST. TELECONNECTIONS  
RELATIVE TO THE POSITIVE HEIGHT ANOMALIES ASSOCIATED WITH THE  
PACIFIC RIDGE PROVIDE GOOD SUPPORT FOR THE CONSENSUS TROUGH  
POSITION. AS THIS PATTERN BECOMES ESTABLISHED THE UPPER TROUGH  
INITIALLY OVER THE EAST WILL FILL/LIFT OUT, AND BY NEXT SUN MEAN  
RIDGING (WITH PERHAPS AN EMBEDDED WEAKNESS OR TWO) MAY EXTEND FROM  
THE WESTERN HALF OF THE ATLANTIC ACROSS THE SOUTHERN HALF OF THE  
EASTERN/CENTRAL U.S. THROUGH THE SOUTHERN ROCKIES AND NORTHWESTERN  
MEXICO.  
 
FROM DAY 3 WED INTO DAY 5 FRI AN OPERATIONAL MODEL BLEND  
CONSISTING OF THE 00Z GFS/ECMWF/UKMET/CMC EITHER REPRESENTED  
CONSENSUS OR PROVIDED A GOOD INTERMEDIATE STARTING POINT DEPENDING  
ON THE FEATURE. THE 06Z GFS WAS OUT OF SYNC WITH CONSENSUS  
(INCLUDING THE 00Z FV3 GFS AND TO A REASONABLE DEGREE LATEST GEFS  
MEANS) FOR FLOW DETAILS OVER THE EASTERN HALF OF CANADA, LEADING  
TO A MORE AMPLIFIED AND PERSISTENT UPPER TROUGH OVER THE EAST BY  
LATE IN THE WEEK. THUS THE 06Z GFS WAS OMITTED FROM THE FORECAST  
BLEND. EVEN AMONG REMAINING SOLUTIONS THERE IS STILL A FAIR  
DEGREE OF SHORTWAVE SPREAD WITHIN THIS OVERALL WEAKENING/DEPARTING  
TROUGH. MEANWHILE GUIDANCE HAS YET TO LOCK ONTO SPECIFICS OF THE  
WEAK TROUGH/UPPER LOW INITIALLY OVER THE NORTHWEST. LATEST  
CONSENSUS IS GENERALLY EAST OF THE PREVIOUS CYCLE AT THE START OF  
THE PERIOD EARLY WED. STILL THERE IS FAIRLY GOOD AGREEMENT AS THE  
EJECTING SHORTWAVE REACHES THE NORTHERN PLAINS/UPPER MS VALLEY BY  
FRI. CONTINUITY/CONSENSUS REMAIN GOOD FOR THE UPPER TROUGH  
REACHING WESTERN CANADA AND THE NORTHWEST U.S. BY THU-FRI.  
 
BY DAYS 6-7 SAT-SUN THE PRIMARY FORECAST UNCERTAINTY CONTINUES TO  
INVOLVE THE SPECIFICS OF HOW UPSTREAM ENERGY ROUNDING THE PACIFIC  
RIDGE REINFORCES THE WESTERN CANADA/NORTHWEST U.S. MEAN TROUGH  
PLUS THE SHORTWAVE DETAILS WITHIN THE PROGRESSIVE  
WEST-SOUTHWESTERLY MEAN FLOW TO THE EAST OF THE TROUGH. THUS IN  
SPITE OF ABOVE AVERAGE CONFIDENCE IN THE MEAN PATTERN, EMBEDDED  
FEATURES HAVE SUFFICIENTLY SMALL SCALE TO SUGGEST BELOW AVERAGE  
PREDICTABILITY. AS A RESULT ASSOCIATED SURFACE WAVE/FRONTAL  
DETAILS ACROSS THE NORTHERN U.S. AND SOUTHERN CANADA WILL LIKELY  
CONTINUE TO VARY IN THE GUIDANCE OVER THE COMING DAYS. OVERALL AN  
AVERAGE OF THE LATEST GFS/ECMWF RUNS AND THEIR ENSEMBLE MEANS  
PROVIDED A GOOD BALANCE OF REPRESENTING THE MEAN PATTERN WITH A  
MODERATE DEGREE OF DETAIL. NOTE THAT THE 06Z GFS REPLACED SOME OF  
THE 00Z GFS INPUT BY DAY 7 AS THE 00Z RUN SIGNIFICANTLY FLATTENED  
THE NORTHERN SIDE OF THE PACIFIC RIDGE WITH MINIMAL SUPPORT FROM  
OTHER GUIDANCE. ALSO THE 00Z CMC AND A NUMBER OF CMC ENSEMBLES  
FLATTENED THE OVERALL PATTERN TO A GREATER DEGREE THAN SUPPORTED  
BY ESTABLISHED CONSENSUS AND TELECONNECTIONS.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
THE COLD FRONT INITIALLY NEARING THE EASTERN AND GULF COASTS WILL  
BE ACCOMPANIED BY LOCALLY HEAVY SHOWERS/THUNDERSTORMS. HIGHEST  
RAINFALL TOTALS ON WED, AT LEAST ON AN AREAL AVERAGE BASIS, SHOULD  
BE OVER THE NORTHEAST WITH INFLUENCE OF STRONGEST DYNAMICS ALOFT.  
THIS FRONT WILL STALL NEAR THE SOUTHEAST/GULF COASTS AND PROVIDE  
MULTIPLE DAYS OF DIURNALLY FAVORED SHOWERS/STORMS OF VARYING  
INTENSITY. PORTIONS OF THE NORTHERN-CENTRAL PLAINS INTO THE  
MIDWEST/UPPER GREAT LAKES WILL LIKELY SEE AT LEAST A COUPLE  
EPISODES OF HEAVY AND/OR STRONG CONVECTION DURING THE LATE  
WEEK/WEEKEND TIME FRAME, FOCUSED BY SHORTWAVES EMERGING FROM THE  
WEST AND SURFACE FRONTS WITH EMBEDDED WAVES. PERSISTENT MONSOONAL  
MOISTURE WILL CONTINUE TO AID IN DAILY CONVECTION OVER THE FOUR  
CORNERS STATES THROUGH THE PERIOD.  
 
RELATIVELY PLEASANT TEMPERATURES AT LEAST SEVERAL DEGREES F BELOW  
NORMAL IN PLACE FROM THE PLAINS TO JUST WEST OF THE APPALACHIANS  
AS OF WED WILL PUSH EASTWARD AND THEN MODERATE AS HIGH PRESSURE  
CROSSES THE EASTERN HALF OF THE COUNTRY. STRENGTHENING SOUTHERLY  
FLOW BY THE LATTER HALF OF THE PERIOD SHOULD BRING WARMEST  
ANOMALIES (5-10F ABOVE NORMAL) INTO THE CENTRAL  
PLAINS/MIDWEST/GREAT LAKES REGIONS. THE EXTREME NORTHWEST SHOULD  
SEE HIGHS COOLING TO A FEW DEGREES F BELOW NORMAL BY FRI-SUN AS  
THE MEAN TROUGH ALOFT SETS UP OVER THE REGION.  
 
RAUSCH  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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